This invention relates to a light-emitting diode array structured so as to enhance output efficiency by reducing internal reflection, and to a method of fabricating such an array.
Light-emitting diode arrays (hereinafter referred to as LED arrays) are employed in, for example, electrophotographic printers and copiers, where they create images by illuminating a photosensitive drum. A conventional LED array is formed on an n-type semiconductor substrate by the following steps. First, one surface of the substrate is coated with an insulating layer. Next, the insulating layer is etched, using a patterned etch mask, to create a plurality of windows below which the substrate is exposed. A p-type impurity is diffused through the windows into the substrate, forming a p-type diffusion region below each window. Finally, electrodes are formed on both surfaces of the substrate.
When current is supplied through the electrodes, light is emitted near the pn junctions between the p-type diffusion regions and n-type substrate. Much of the emitted light exits through the windows and can be used for image formation or other purposes. However, part of the light is lost due to internal reflection at the interface between the p-type diffusion region and air, which occurs because of the great disparity between their refractive indexes, e.g., 3.5 for the p-type diffusion region versus unity for air.
This problem came to the inventors' attention when they measured the emission profile of a single LED in an LED array. FIG. 6 shows the results. A plan view of the LED can be seen at the top of FIG. 6, illustrating the insulating film 12 and one window 14, below which is a p-type diffusion region 16. The p-type diffusion region actually extends out to the dotted line in FIG. 6, so it includes a side area 20 disposed below the insulating film 12. An electrode 22 makes contact with part of the p-type diffusion region 16. Current was fed through this electrode, and the intensity of emitted light was measured along line D--D.
The graph at the bottom of FIG. 6 shows the general form of the intensity profiles obtained in this way. Intensity is shown on the vertical axis and position on the horizontal axis, matching position on the above-mentioned line D--D. Even though light emitted from the side areas 20 of the LED had to travel through the insulating film 12, a greater intensity was measured over these side areas 20 than over the window 14. If the maximum intensity over the side areas is 100%, the intensity over the window 14 was about 20% less.
This result could be explained if the insulating film 12 were acting as an anti-reflection coating. Other possible explanations involve the depth of the pn junction, which is greater under the window 14 than in the side areas 20. In any case, it would clearly be advantageous to obtain a 100% intensity profile across the entire surface of the LED, and this can be done by reducing internal reflection in the window 14.